It's your monitors active backlight. When it knows its displaying a lot of very dark areas the backlight will dim. This can be turned off it its settings, so make a user profile and turn it off and set the colors the way you like em.

Yeah, this is an artificial way to boost contrast ratio (not a particulary good one). I have the same thing on my Samsung. Easiest way to test this is to have 1 fully white image and 1 fully black one. Then maximize both of them on desktop and switch between them and observe window frame and Windows taskbar. If the brightness of those elements changes when you switch between both images, the monitor is doing that.